Automatic telephone system



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1,550,801 c. L. GoonRuM AUTOMATIC TELEPHONE SYSTEM Filed Feb. 17, 1920'I Shouts-Sheet 3 /ni/fw for: Ww/fs L, bad/um by Affy.

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C. L. GOODRUM AUTOMATIC TELEPHONE SYSTEM Filed Fab. 1'?, 1920vsheets-sheat 4 /H van for.' Vzw/as l.' Gow/mm l Amig. 25, 1925.

C. L. GOODRUM AUTOMATIC TELEPHONE SYSTEM Filed Feb. 17, 1920 7Sheets-Sheet 5- Aug. 25, 1925.

1,550,801 C. L. GGODRUM AUTOMATIC TELEPHONE SYSTEM Filed Feb. 17, 192C'7 Sheets-Sheet 6 I 4/rf/ vak/fak [7Min/f5 Wd/fam Aug. 25, 1925. l1,550,801

C. L. GOODRUM AUTOMATIC TELEPHONE SYSTEM Filed Feb. 17, 1920 l. '7Sheets-Sheetl 7 IlT- /h van/0n y (har/@s L .Cdad/w71 by lfm vAbw.-

Patented Aug. 25, 1925.

UNITED STATES PATENT OFFICE.

CHARLES L. GOODRUM, 0F NEW YORK, N. Y., ASSIGNOR T0 WESTERN ELECTRICCOM: PANY INCORPORATED, OF NEW YORK, N. Y., A CORPORATION 0F NEW YORK.

AUTOMATIC TELEPHONE SYSTEM.

Application filed February 17, 1920. Serial No. 359,327.

T0 all whom, t may concern.'

Be it known that I, CHARLEs L. GooDRUM, a citizen of the United States,residing at New York, in the county of New York and State of New York,have invented certain new and useful Improvements in Automatic TelephoneSystems, of which the following is a full, clear, concise, and eXactdescription.

The present application is a continuation in part of my applicationSerial Number 329,922, filed OctOber 1l, 1919.

This invention relates to telephone systems wherein connections betweenlines may be established under the control of the subscribers andwithout the intervention of operators.I

I-Ieretofore such so-called automatic telephone systems employed eithermachineswitching means or combinations of great numbers of relays.

The object of this invention is to produce a system employing a smallnumber of relays per line in which all of the functions performed inmodern systems using machineswitching means will be eflicientlyperformed.

Through the use of novel circuit arrangements and a novel method ofselection, the above object has been accomplished.

The present invention contemplates a system in which the method ofselection of a path to a called substation simulates the method used inmodern telephone practice, that is to say, a large group of lin-es isserved by a small group of trunks, the proportion of one to the otherbeing determined by the traffic condition. The calling line is extendedover a seized trunk to a particular large group of lines, whereuponanother trunk is seized and over this the line is further eX- tended toa particular subgroup ofthe selected large group and, thereafter, aparticular line is selected. So far the similarity is complete.

By using mechanism individual to each line it has heretofore beenpossible to obviate trunk-hunting by preselecting a trunk and holding itready for the first incoming call and by using a common mechanism it hasheretofore been possible to reduce the number of pieces of mechanism.The present invention contemplates an arrangement which combines theadvantages of both sysi tems; a system in which a common mechanism isemployed and in which an idle trunk is preselected and held ready forthe first incoming call. In other words, instant connection to an idletrunk is accomplished through the use of a common mechanism and thisapplies not only to the selection of a trunk upon the initiation of acall, but to the selection of a trunk after the first digit selectionhas been made and so on.

In accordance with this invention, each line is provided with the usualline and cutoff relay, but the line relay performs a new function.I-Ieretofore this piece of apparatus merely functioned to initiate themovement of other apparatus and the calling line was identified throughthe method of changed potential on the third or test conductor in asystem employing a common mechanism to respond to the calling lines,such for instance, as the well-known line finder switch system. The linerelay in this system energizes connecting mechanism to instantly connectthe calling line to the preselected trunk, thus being employed not onlyto startithe common mechanism in motion, but also to identify thecalling line.

When the call has been extended over a trunk and the irst digitselection has been made, a pair of contacts are closed which performfunctions similar to those of the line relay to actuate the next stagecommon mechanism and to identify the calling trunk.

In a ten per cent four-digit system there would be available for the useof a group of 100 lines, 10 trunks. According to the present inventioneach of these trunks leads to ten selecting mechanisms each serving adifferent group of 1000 lines. Entering each of these selectingmechanisms will be 100 trunks in groups of l0, incoming from l() groupsof 100 lines each. Leaving each of these selecting mechanisms will be 10trunks each leading to ten similar selecting mechanisms which, in turn,each serve a different group of 100 lines. In a similar manner each ofthese mechanisms have 100 incoming and 10 outgoing trunks. These lastmentioned ten trunks each go directly to a selecting mechanism leadingto a particular 100 lines.

In a ten per cent three-digit system the arrangement will be the samewith the exception that one of the selecting mechanisms will be absent.Therefore, each of the trunks leading from the mechanism wherein thecalling lines are identified will lead to a selecting mechanism in turnleading to a different group of 100 lines. Entering this Selectingmechanism will be 100 trunks in groups of ten incoming from l() groupsof 100 lines each. Leaving each of these mechanisms will be l() trunkseach leading to a selecting mechanism having direct control over a groupof 100 lines.

While it has been found in commercial practice that the ten per cent.system gives the best results, this invention is not limited to such asystem.

A calling line is identified through the method of electricallydistinguishing two circuits one each in the principal direction of a twocoordinate system, and through these circuits connecting mechanisms areinstantly energized to connect the line thus identified to an idlepreselected trunk. rlhereupon the subscriber operates a calling deviceto actuate the mechanism included in the trunk whereby one of the lObranches of this outgoing trunk is identified in a. particular one ofthe Al0 selecting mechanisms to which it leads. The identification ofthe trunk is accomplished in a similar manner to the identification ofthe calling line. rlhis action is then repeated in the second selectingmechanism after the second stage selection has been made by the callingsubscriber. The mechanism in the seized trunk between the second stageselecting mechanism and the selecting mechanism leading to theparticular 100 lines out of which one is to be selected7 is actuated bythe calling subscriber. Here the identification of the called line ismade through the method of electrically distinguishing two circuits eachin the principal direction of a two coordinate system, the onlydifference being that in the previous identifications made the twocircuits were electrically distinguished simultaneously while in thislast stage the two circuits are electrically distinguished one after theother and in response to directive operations on the part of thesubscriber. rather than automatic operations on the part of the callingline or the calling trunk.

The means for identifying the calling line comprises contacts on theline relay. The means for identifying the calling trunk comprisescontacts controlled over the trunk and having` a similar function to thecontacts on the line relay. Such contacts associated with the callingtrunk may be actuated either by a small step-by-step switch or by acombination of relays known in the art as count-- ing relays. Thequestion of preference between the two methode of actuating contacts ismerely a question of commercial feasibility and hence both methods areillustrated and described in the following specification.

The method of preselecting an idle trunk entails no limiting actionwhatsoever and occupies only that amount of time which it tal-:es anelectrical relay to become energized. The selection of idle trunks isjtherefore, to all intents and purposes instantaneous and as soon as onetrunk is seized another is preselected and ready for instant use.Therefore, a plurality of calling lines may all be served practicallywith no loss of time. Upon the seizure of the first idle trunk a relayis actuated to preselect the next idle trunk and if a second callingline immediately initiates a call it will seize the second preselectedtrunk and in turn a third trunk will be preselected through the mereenergizing of a relay associated with the second trunk. Since it isalmost impossible for two lines o initiate a call at exactly the sameinstant there will be no probability of two lines seizing the sametrunk.

The release of the connection is under the control cf the callingsubscriber. lVhen he replaces his receiver on its hook, release of allselected mechanism involved in the connection will be instantaneoussince it inif'olves only the return to normal of connecting mechanismshaving no movement eX- cept that from a normal to an actuated posi--tion or vice versa.

ln the drawings, Fig. l is a diagrammatic view of the trunkingarrangement employed. F ig. 2 diagrammatically represents the selectingmechanism by means of which a calling line is connected to an idletrunk. Fig. 3 diagrammatically represents the distribution of the trunksto the selecting mechanisms leading to different groups of lines. Fig.diagranmiatically represents selecting` mechanisms by means of which thebeforenientioned trunks are connected with trunks leading to selectingmechanisms having functions similar to those of final selectors orconnectors. Fig. 5 diagrammatically represents the selecting mechanismswhich are equivalent to the final selectors or connectors, and throughwhich connection of a trunk to a called subscribers line is made. Fig.(l diagrammatically represents a detail of F 8 showing the identifyingmeans by which a trunk identifies itself in the next stage selectingmechanism. Fig. 7 diagrammatically represents a detail of Fig. 3 and isto be used in connection with Fig, 8. Fig. S is a detail view whichdiagrammatically represents a portion of Fig. 5. lt is an organizationof circuits by means of which the calling subscriber controls theselecting mechanisms which are equivalent to the final selector orconnector to make the final connection to the called subscribers line,actuate his call bell and perform other functions vi'ttt13991131'.

ordinarily performed in such a mechanism. Fig. 9 is a schematicrepresentation of the release circuit of an established connection.

The general trunking arrangement is shown in Fig. 1. The square numbered102 represents the connecting mechanism into which lines numbered 0 to99 enter and out of which trunks, which, for convenience, we will number0 to 9, lead. n a similar inanner square 103 represents the connectingmechanism into which lines to 199 enter and from which trunks 10 and 19leave. In aI fully equipped system there will be ten of these groups oitmechanisms serving lines 0 to 999 inclusive, and out of which trunks 0to 99 lead. Trunks 0 to- 99 then go into connecting mechanisms in thesame manner. For instance, square 104 represents a connecting mechanisminto which trunks 0 to 99 enter and out of which trunks which, forconvenience, we will number 1000 to 1009, lead. These trunks 0 to 99also enter nine other such connecting mechanisms, of which 105 and 106are shown. A calling subscriber in the group, say 103, would beconnected to a trunk in the group 10 to 19. If this calling subscriberdesired another subscriber in the rst thousands group he wouldmanipulate his calling dial to identiy this trunk in that one of the tenmechanisms into which his trunk enters which leads to the large group oflines numbered 10004999. 7e will assume that 104 is the desired one andthat by sending one impulse, he has identified the trunk to which he hadbeen connected in the group 104 and thereby selects a trunk in the group1000 to There are ten such mechanisms as 104 intowhich trunks 0 to 999enter and out of which trunks 1000 to 1099 lead. The trunks 1000 to 1099enter ten mechanisms similar to 104, one of which is here shown numbered107. The subscriber, having picked out a line in the first thousandsgroup, now wishes to select a line in the lirst hundreds group of thefirst thousands group by selecting the first of the mecha.- nisms thatis numbered 107. By sending one impulse he is connected to one of agroup of trunks represented by 108. This trunk takes him into aconnecting mechanism 114 serving lines 1100 to 1199.

With this short description and with the aid of the numbering on Fig. 1,the entire layout will be plain.

A similar view of a three-digit system may be obtained by calling thetrunks 0 to 99 entering 104, lines 0 to 99 and the trunks 100 to 199entering 109, lines 100 to 199 and the trunks 200 to 299 entering 113,lines 200 to 299, etc., and disregarding the remainder of the figure tothe right. Trunks 1000 to 1009 would then become trunk 0 to 9 and thegroup of trunks 1000 to 1099 would become a group of trunks 0 to 99.Mechanism 114 would then serve lines 100 to 199, there being nothousands digit.

A similar diagram for a live-digit system could be built up by callingthe lines 0 to 99 entering 102 trunks 0 to 99 and providing a similarlayout.

The following specification is based on a three-digit system rather thana four-digit system, as this smaller system will illustrate sucientlyevery ifea-ture of this invention.

In order to facilitate the description, only a small number of incominglines and outgoing trunks are shown. It will be understood, however,that the incoming lines and the number of outgoing trunks are unlimited.Let it be assumed that this system is designed on the decimal basis andthat the trunking arrangement is what is known as the 10 per centsystem. Let it be further assumed that the subscribers lines shown inFig. 1 belong to that group of lines numbered 200 to 299 inclusive. Sixof these lines have been shown, 210 to 212 inclusive, and 220 to 222inclusive. The invention will be illustrated by describing theestablishment of a connection between line 220 and line 112.

When line 220, as numbered on the drawing Fig. 2, initiates a call, linerelay 1 is energized. Through its armature it places ground potential ontwo circuits, each in the principal direction of a two coordinatesystem. The lirst group of circuits 2 is associated with the tensdesignation of the calling lines and the group 3 is associated with theunits designation of the calling lines. Thus all the line rela-ysassociated with lines 220 to 229 inclusive, upon actuating theirarmatures, will ground circuit 5 in group 2. Thus, also, line 220 willground circuit 8 in group 3; line 221 will ground circuit 9 in group 3;line 222 will ground circuit 10 in group 3 and so on. Returning to thefunction of line relay 1, it will be seen that circuits 5 and 8 aregrounded. Following these circuits as they lead to the right and upwardsit will be seen that relay 12 is energized.

The group of relays 12, 22 and 23 individual to the trunk 27 is one of anumber of similar groups employed for the purpose of preselecting atrunk. As will appear from the following description, the fast relay 12and slow releasing relay 23 of a preselected trunk are actuated, relay22 of a busy trunk is actuated and all relays of idle trunks aredeenergized. Thus the trunk represented in the group of rela-ys belowthe one being considered is represented as being a busy trunk and itsrelay 26 (corresponding to relay 22) is energized. The energization ofrelay 26 causes the energization of relays 12 and 23 of the next idleltrunk and this puts the trunk 27 in the preselected condition, hence,upon the energization of the line iii- relay 1, the. circuits Yi'or theenergization of' the groups and sub-group relays are cempieted throughthe armatures and contacts of relay Through Vthe.corresponfclinglett-hand armature ot relay 12, circuit will be grounded,which will, in turn, establish a circuit for relays 13 and 22. Thiscircuit may be traced from ground at the armature and innermost contactol` relay 1, circuit 5, corresponding lett-hand armature and contact orrelay 12, circuit 15, winding ot relay 13, winding oit relay 22, tobattery and ground. Tiel-ay 13 locks up in a circuit extending lfromground, the arn'iature and contact of relay 23, conductor 24, right-handarmature and Contact of relay 13, and thence over the circuit previouslydescribed to battery and ground. rt the same time through t-he outercontact ot relay 1 a circuit is established which may be traced fromground, the armature and outer contact of relay 1, circuit 3,corresponding` right-hand armature and contact ot relay 12, circuit 18,winding of relay 25, winding oi relay 22, to battery and ground. Relay25, in a similar' manner to relay 13, locks up to the ground onconductor 2li. Relay 22 being energized with relays 13 and 25 moves itsarmature from its back to its front contact. lt should be stated at thispoint that previous to the energization ot relay 22, a` circuit wasnormally established 'from ground, the armature and iront contact ofrelay 2G associated with a busy trunlr, the armature and back contact otrelay 22, and thence in parallel through the windings of relays 12 and23. Upon the energization oi' relay 22, relay 12 becomes deenergizefcl,but relay 23 being slow to release, does not immediatelyv remove theground from conductor 2a. The purpose oi this is to allow a sufficientinterval tor trunk 2?, associated with relay to return ground fromanother point to hold relays and 13 energized. The energization ofrelays 13 and 25 will connect line 220 to trunk 27, through the outerset of three armatures and contacts ot relay 13 and the right-hand setof contacts oit relay 25. Upon the establishment of this circuit, theground on conductor 2li will be extended to conductor 23 through innerright-hand armature and iront contact oif relay 25 and thence to thethird wire of line 220 through the inner armature of the outer set ofthree armatures of relay 13 where it will energize cutoff relay 29.

Connection has now been established between line 220 and trunk 27 andground is placed on the conductor 24 by the liront contact and armatureof relay 23 to hold relays 13, 25 and 29 energized, until ground isreturned on conductor 211 from trunk 27. Relay 22 having becomeenergized, relay 12 becomes deenergized as described, and in turn relays3() and 31, associated with the rascher next trunlr 32, become energizedthrough the transicr of the ground at the armature of relay 26 to thewindings of relays 30 and 3 Associated with each trunk, as shown in Fig.3, is a mechanism indicated by squares 33, 34 and Square 34 isassociated with trunk 27 and the mechanism represented ther-cat is shownin Fig. G. llt will be seen that when the talking conductors of trunk 2Tare connected to the calling substation, relay 33 will be energized.This relay, in moving its a mature trom its back to its 'liront contact,energizes a slow-releasing electroinagnet 3-1 which, besides controllingthe holding pawl ot a small step-by-step switch, also controls twoarmatures. At the outer lett-hand one ot' these armatures, a ground isplaced on the lead 24 to replace the ground hold on this lead at thearmatin'e ot slow-releasing relay The connection is now in a stable`condition and operations arc suspended until the subscriber on line 22()manipulates his calling device.

it was assumed that line 220 was to establish a call to line 112. Thesubscriber, theretore, manipulates his calling device to send a singleimpulse over the connection. This impulse is in the form of aninterruption of a line circuit in accordance with modern automatictelphone practice. This single impulse deenergizes relay 33 during asufficient interval to establish a circuit from the armature and backcontact of relay 33, the inner left-hand armature and Contact of relay34, the winding of stepping magnet 35 to battery and ground, whereuponthe brushes 36 and 37 are stepped on to their first contacts.

Before going further with the description ot the operation, it will benecessary to explain F 3 more in detail. ln this ligure, trunk 27 isshown with three sets of taps, 3S, 39 and 490, these beingrepresentative of the ten taps which would be used in a ten per centsystem as assumed. At the righthand portion of Fig. 3, two squares, l1and l2, are shown. These squares represent lineconnecting mechanismsimilar to that shown in Fig. 2. The square l1 representing theconnecting mechanism associated with the lines whose members are 0 to 99inclusive, and the square 112 representing the mechanism associated withthe group of lines whose members are 100 to 199 inclusive. The set oftaps 33 ot the trunk 27 leads to a connecting mechanism which in turnleads to a group of lines numbered 100 to 199 inclusive. The set of taps39 leads to a mechanism which in turn leads to a group of lines numbered200 to 299 inclusive, and in a similar manner seti of taps 40 leads to amechanism which in turn leads to a group of lines numbered 300 to 399,inclusive.

In further explanation of this system,

and referring to Figures 2, 3, 4 and 6, it

' may be stated that each trunk leading from a non-numerical switchingmechanism such as is shown in Figure 2, is provided with a connectingmechanism actuating means indicated in Figure 3 and shown in detail inFigure 6. This means comprises 'two brushes, 36 and 37, arranged totravel over and make contact with sets of terminals connected to twosets of conductors arranged in the two principal directions of a twocoordinate system. The brushes 36 and 37 move together so that they mayboth be placed on their first terminals connected to 45 and 51, on theirsecond terminals connected to 59 and 60 or on their third terminalsconnected to 61 and 62. Thus, by one, two or three steps of the brushes36 and 37 the trunk 27 may be identified in the non-numerical switchingmechanism shown in Figure 4, non-numerical switching mechanism II inFigure 3 or non-numerical switching` mechanism III in Figure 3.

It should be noted that no two of the connecting mechanism actuatingmeans' shown in Figure 3 are connected in the same manner. The set ofbrushes shown directly above 36 and 37, Figure 3, connect through theirfirst terminals, not with 45 and 51 as at 36 and 37, but with 45 and theconductor to the left of 51. The set of brushes shown directly below 36and 37 connect through their first terminals with 45 and the conductorto the right of 51. The set of brushes to the right of 36 and 37 connectthrough their first terminals with the conductor to the right of 45 andwith 51. lVe thus have a connecting mechanism actuating means individualto each trunk, so associated with the sets of contacts leading to thedifferent non-numerical switching means to which the trunks lead, thateach trunk may be identied in each non-numerical switching means throughthe method of electrically distinguishing two contacts, one each in theprincipal direction of a two coordinate system.

The system may be readily explained in terms of a three coordinatesystem. Thus a set of trunks as 0 to 99, each having a single point ofingress as at 102, 103 and so forth, and each having a plurality ofpoints of egress as at 104, 105, 106 and so forth, are arranged to beidentified at their points of egress by conductors such as 45, 51, 59,60, 61, 62 and so forth arranged in a three coordinate system. Eachtrunk might be said to be on a different line in one of the threeprincipal directions of such a system, and as the pointers (the brushes36 and 37) are moved one, two or more steps in that direction, theycontact with two contacts one each in the remaining two principaldirections of the three coordinate system.

Since the subscriber on line 220 has manipulated his sender to transmita single impulse, brushes 36 and 37 will come to rest on their firstterminals. Stepping magnet 35 is so constructed that its armature whichcontrols the ratchet wheel 43 is quick to act but the armature shown atthe left-hand end being of much lighter construction, and under a muchsmaller spring pressure, is slow to release. After the singleinterruption of the circuit, relay 33 attracts its armature and holds itin this position thereafter. Magnet 35 becoming deenergized after ashort interval, the left-hand armature retracts and closes a circuitfrom ground, the outer left-hand armature and contact of magnet 34',conductor 24, the outer left armature and contact of relay 44, thelefthand armature and two contacts of magnet 35 and thence in parallelto the brushes 36 and 37. In this manner two conductors each in theprincipal direction of a twocoordinate system are momentarily grounded.The conductor grounded by brush 36 and numbered on Fig. 3 as conductor45, leads through a corresponding left-hand armature and contact ofrelay 46 to conductor 47, relay 48, conductor 49, winding of relay 50 tobattery and ground. The conductor grounded by brush 37 and numbered onthe drawing 51, leads through a corresponding right-hand armature ofrelay 46, to conductor 52, winding of relay 53, conductor 49, winding ofrelay 50, to battery and ground. Thus through a movement of one step bybrushes 36 and 37, relays 48 and 53 are energized and the trunk 27 iseX- tended through a middle set of three armatures and contacts of relay48 and through the set of three right-hand armatures and contacts ofrelay 53 to a trunk 54. In a manner similar to that heretoforedescribed, conductor 55 is temporarily grounded by the left-handarmature and contact of relay 56 until a ground has been returned fromconductor 55 of trunk 54. Also in a similar manner to that heretoforedescribed, relay 50 becomes energized and transfers the ground at itsarmature to the windings of relays 57 and 58 of the neXt idle trunk.

If the relay 33 had been deenergized twice to cause the stepping magnet35 to move the ratchet 43 two steps, then brushes 36 and 37 would haveeach been stepped to their second contacts. In this case conductors 59and 60 would have been grounded and consequently relays in the mechanismentered by tap 39 of trunk 27 would have been energized. Also, in asimilar manner, if brushes 36 and 37 had been given three steps,conductors 61 and 62 would have been grounded and relays in themechanism entered by tap 40 of trunk 27 would have been energized.

Upon the seizure of trunk 54, ground on conductor 55 will be extendedthrough the DIlGI fight-hand armature of relay 53, third Wire conductor621 corresponding armature and contact of relay 48, third wire conductorof trunk 27, winding of relay 44 (Fig. 6) to battery and ground. Relay44 in energizing extends the ground from conductor 55 through its innerlett-hand armature and contact to conductor 24 and at its right-handarmature opens the circuit of relay 33. Relay 33 becomes deenergizedand, after an appropriate interval, magnet 34 in turn becomesdeenergized. Upon its deenergization, magnet 34 retracts the holdingpawl 63 and the brushes 36 and 37 are returned to normal. Thus brushes36 and 37 are similar in function to the Contacts on the line relay 1,and the relay 44 is similar in function to cut-ott relay 29.

Trunk 54 leads through a combination of relays represented by square.64. This is shown more in detail in Fig. 8.

In Fig. 8 it will be seen that when trunk 54 is seized by the line 220,line relay 65 will become energized. This relay, through its left-handarmature and front contact, energizes sl-ow-to-release relay 66, which,through its right-hand armature, grounds conductor 55, thus replacingthe ground at the armature and contact of relay 56. At its outerleft-hand armature relay 66 places a ground on conductor 155. rlheconnection is nowl again in a stable condition and awaiting the furtheroperations of the calling subscribers sending device.

It was assumed that subscriber on line 220 would establish a call toline 112, and it has been seen how through the sending of a. singleimpulse, that subscriber has selected a trunk leading to a group oflines all ot whose 100s digits are one.

The calling subscriber now manipulates his sending device to interruptthe circuit of line relay a single time. Upon the retraction of thearmature of relay 65 a circuit is established from ground, left-handarmature and back Contact ot relay 65, inner lefthand armature andcontact of relay 66, inner left-hand armature and contact of relay 68,conductor 84, winding of relay 67, conductor 83, outer left-handarmature and contact of relay 71, outer right-hand armature and contactof relay 72 to battery and ground. Relay 67 becomes energized andthrough its right-hand armature and front contact places a ground on oneterminal of relay 69 whose other terminal is connected through conductor83 to battery and ground, whereby relay 69 is energized. The groundplaced on conductor 84 while the armature of relay 65 is resting on itsback contact is also extended through the letthand armature and backcontact of relay 75, the winding of relay 76 to battery and ground.Relay 76 becomes energized and by means of its left-h and armature andcontact extends a circuit from ground, the outer letthand armature andcontact of relay 66, the left-hand armature and contact of relay 67conductor 81, the said armature and contact of relay '7 6 to oneterminal of relay 75. Relay 75, however, is shunted at this time by theground on conductor 84. Upon the attraction of the armature of relay 65immediately after the interruption of the circuit of relay 65, the shuntaround the winding of relay is removed and it becomes energized inseries with relay 76. Since there is only a single interruption of theline circuit for transmitting the digit 1, only the pair of countingrelays 75 and 76 will become in this manner energized. The conductor 77which is one of the conductors in the two groups of conductors each inthe principal direction of a two-coordinate system, is now connectedthrough the right-hand contact and armature of relay 76 and the backcontacts and righthand ari'nat-ures ot the other lower coimting relaysto conductor 80. A short interval after relay 65 has become energizedrelay 67 retracts its armatures. Thereupon a circuit is closed fromground, the right-hand armature and contact of relay 67, the innerright-hand armature and front contact oi? relay 69, the outer right-handarmature and back Contact of relay 7 9, conductor 80 and thence toconductor 77. Conductor 77 is connected to relay 78 and the grounding otthis conductor causes the energization of relay 78 which through itsright-hand armature and contact locks up to the ground on conductor 155supplied by the Yfront contact and outer left-hand armature of relay 66.

After relay 67 has become deenergized, and before relay 69 becomesdeenergized, ground is extended to relay 70 the other terminal ot' whichis connected to battery. Relay 70 becomes energized and through itsarmature and contact extends ground through the normal lett-hand contactand armature ot relay 79. The other terminal ot relay 79 is, however,connected to conductor 155 and as this constituted a` shunt circuitrelay 79 does not at this time become energized.

Shortly after relay 67 becomes deenergized, relay 69 retracts itsarmatures. At its outer right-hand armature it removes the ground 'fromconductor 81 whereby the counting relays are released. At its innerright-hand armature it removes ground from the winding of relay 70 butthe latter holds up momentarily in series with relay 7 9. The latterenergizes and by so doing' at .first shunts relay 7 0 through thelefthand continuity contacts of relay 79 and then opens the circuit ofrelay 70, whereby it becomes deenergized. Relay '79 is now heldenergized over the following circuit: ground, outer left-hand armatureand contact or' relay 66, conductor 155, winding of relay 79, left-handarmature and alternate contact of relay 7 9, resistance 98, conductor83, outer left-hand armature and back contact of relay 71,` outerright-hand armature and contact of relay 72 to battery and. ground.

The apparatus is again in a stable condition awaiting the receipt of thelast series of impulses.

In accordance with our previous assumption, the subscriber will now senda train of two impulses. Having picked out a group of lines whosehundreds digit was 1 and Whose tens digit was 1, the calling subscriberwishes to pick out a particular one of these lines Whose units digit is2 in order to connect with the line numbered 112. Two impulses will besent and relay will be deenergized twice in quick succession. Upon thefirst retraction of its left-hand armature, the relay 67 will becomeenergized, as previously described. Upon the first interruption, thecircuit for relay 76 will again be established and relay 76 will becomeenergized. At the end of this interruption, this circuit will be openedbut relay 76 in the meantime having attracted its leftehand armature, itwill remain energized in series with the winding of relay 75, asheretofore described. Relay 75 becomes energized and transfers thecircuit of conductor 84 through its left-hand armature and frontcontact, the left-hand armature and front contact of relay 86 to thewinding of relay 87. Upon the beginning of the second impulse, relay 65closes the circuit heretofore described which now includes the windingof relay 87 whereby relay 87 becomes energized. Shortly thereafter thecircuit including conductor 84 is opened at the left-hand armature andcontact of relay 65 and the shunt being removed from around relay 86, itbecomes energized in series with the winding of relay 87. Conductor 89is now included in a circuit including the front contact and right-handarmature of relay 86, the back contacts and rightehand armatures of therest of the upper row of counting relays to the conductor 82. During thesending of these two impulses relay 67 has become energized through acircuit from ground, the armature and back contact of relay 65, theinner left-hand armature and front contact of relay 66, the innerleft-hand armature and back contact of relay 68, conductor 84, windingof relay 67 condactor 88, back contact and outer left-hand armature ofrelay 71, back contact and outer right-hand armature of relay 72 tobattery and ground. As before, relay 67 causes the energization of relay69. Shortly after the sending of this last :series of impulses relay 67retracts its armatures. A circuit is now established from ground, theright-hand armature and back contact of relay 67, the inner right-handarmature and contact of relay 69, outer right-hand armature and frontcontact of relay 79, conductor 82, and thence to conductor 89. Conductor89 is one of the conductors in the second group of conductors each inthe principal direction of a two coordinate system, and is connected tothe winding of relay 90. Relay 90 is energized and in attracting itsarmatures locks itself through its left-hand armature and contact to theground on conductor 155.

The ground from the armature and back contact of relay 67 is alsoextended to the winding of relay which becomes energized. Relay 70 inattracting its armature entends this ground through the inner righthandarmature and contact of relay 79, the left-hand armature and normalcontact of relay 68 to the Winding' of relay 68, but since the otherterminal of relay 68 is connected to the conductor 155 it is shunted anddoes not energize at this time. A short interval after thedeenergization of relay 67, relay 69 retracts its armatures. The firsteffect of this is to remove the ground from conductor 81 whereby thecounting relays are restored tol normal. Next the ground is removed fromthe terminal of relay 70 but it remains energized in series with relay68 which now that the shunt is removed becomes energized. It is heldenergized through a circuit from ground, the outer left-hand armatureand contact of relay 66, conductor 155, winding of relay 68, right* handarmature and alternate contact of relay 68, resistance 99, conductor 83and thence to battery and ground over the path previously described. Theenergization of relay 68 causes the deenergization of relay 70.

Let it be assumed that line 112 is, at this time, involved in aconnection and is therefore in a busy condition. This busy conditionwill be denoted by a ground on the third conductor which will beextended through the innermost left-hand armature and contact of relay78, the innermost righthand armature and Contact of relay 90, conductor91, the left-hand armature and front contact of relay 69 (before relay69 becomes deenergized), the inner rightnhand armature and normalContact of relay 71, winding of relay 71 to battery and ground. Relay 7lbecomes energized and locks itself through its inner right-hand armatureand alternate contact to conductor 155. By attracting its outerleft-hand armature, relay 71 removes battery and ground from conductor83 whereby relays 68 and 79 are deenergized, or more strictly speaking,relay 79 is deenergized and relay 68 is prevented from being energizedas above described. Through its inner left-hand contact relay 71connects the source of busy tone 92 to one winding of the calledsubscribers battery feed coil whence by induction this tone istransmitted to the calling subscriber.

Attention is now directed to Fig. 9. 1t will there be seen that relay G5through its right-hand armature and 'liront contact controls theslow-releasing relay (5G which, in turn, at its contacts and armatures,places a ground on com luctor and conductor 155 to hold the relayscontrolling the connection in an energized condition. 1t will be seenthat ground on conductor 155 holds relay 71 in an energized condition.1t also holds relays 78 and 90 oitl F ig. 5. Ground on conductor 55, inturn, holds relays 4S and 53 in parallel and relay 5() in series withthese two energized, in Fig. 4. 1t holds relay 44 et Fig. 6 energizedand through the innermost lett-hand armature and front Contact of relay44, this ground is extended to Conductor 24 where it holds relays 13 and25 in parallel with relay 22 in series energized and also holds thecutoi'l' relay E29 energized. 1f the calling subscriber hea-rs the busytone connected to the line through the agency of relay 71, he willreplace his receiver on its hook and therethrough relay will becomedeenergized. After a very short interval, relay 66 will becomedeenergized and by removing the ground from conductors 55 and 155, willcause the instant deeuergization of all the relays just describedwhereupon the connection will be disestablished and all connectingmechanisms returned to their normal. condition.

le will assume now that the line 112 is idle in which case no groundwill be returned over conductor 91 to energize relay 71, as described.Therefore, atter relay 69 has become deenergized, relay 68 becomesenergized in the manner heretofore described. Thereupon a` circuit iscompleted from ground, the outer right-hand armature and bacl contact otrelay 71, the left-hand armature and back contact of relay 69, conductor91, the innermost right-hand contact and armature ot relay 90, theinnermost left-hand contact and armature of relay 78, the winding' ofcutoi'l relay 101 to battery and ground, whereby line 112 is marked asbusy. At the same time, a circuit is established from ground, the outerright-hand armature and contact of relay 69, the outer left-hand contactand armature of relay G8, the winding ot relay 7 4, the outer left-handarmature and contact of relay 71, the outer right-hand armature andcontact of relay 7 2 to battery and ground. Relay 74 attracts itsarmatures and connects a source of ringing current 97 through thewinding of a rnging cutoli relay 94 to the line of the calledsubscriber. The current in this circuit is insuiiieient to cause theenergization et' relay 94 until the subscriber in answer to the summonsof his` bell removes his receiver 'from its hook. Thereupon the flow ofringing current is increased and relay 94 becomes energized.

1t will be noticed that prior to the energization ot' relay 94 thatground from conductor 155 was extended to one terminal ot relay 72, theother terminal of which is connected through resistance 190 to batteryand ground. However, ground from conductor 155 also extends through thecontact and armature of relay 94 to the other terminal et relay 72,whereby it is shunted until relay 94 becomes energized. lllhen thisoccurs, relay 72 becomes energized, and by means of its inner right-handarmature and contact opens the shunt circuit so that it cannot againbecome effective. Through its lefthand armatures and front contacts,relay 72 connects the calling and called subscribers in a conversationalcircuit` Relay 72 at its outer right-hand armature removes battery andground from conductor 83, whereby relays 68 and 79 are deenergized.

Attention is again called to Fig. 9. The release circuit is new in thecondition shown in Fig. 9 with the exception that instead ot relay 71being locked to conductor 155, the ground will extend to the winding ofrelay 72 and relay 71 will hold relay 101 energized by means of theouter right-hand armature and contact ot relay 71. At the end of theconversational period, the calling subscriber, by placing his receiveron its hook, will cause the instant disconnection otl the relaysinvolved in the circuit shown in Fig. 9 in a manner similar to thatheretotore described.

The called line is guarded against intrusion by ground potential fromground at the outer right-hand armature and back contact oit relay 71,lett-hand armature and back contact of relay G9, conductor 91 throughthe corresponding contacts and armatures of relays 90 and 78 to thethird or test conductor or line 112.

Attention is called to Fig. 7 wherein a substitute arrangement for thecounting relays o-t Fig. 8 is shown. This arrangement is very similar tothat shown in Fig. 6 comprising a stepping magnet 98 connected to theconductor S4 and a holding magent 99 connected to the conductor 81. Theimpulse ot current supplied over the conductor 84 causes the steppingmagnet to rotate the wipers ot this small step-by-step switch intoconnection with the contacts, whereby conductor 77 may be grounded overthe conductor 89 by one step and the conductor 89 grounded overconductor 82 by two steps. The operation ot this is obvious when takenin connection with the description of the operation ot the countingrelays.

What is claimed is:

l. In a telephone system employing relays for interconnectingsubscribers telephone lines, a holding and release trunk comprising acontinuous path extending through each stage for maintaining energizedthe interconnecting relays thereat.

2. In a telephone system employing switching devices having permanentlypaired active and passive contacts for interconnecting subscriberstelephone lines, a holding and release trunk comprising a continuouspath extending through each switching stage for maintaining energizedthe switching devices thereat.

3. In a telephone system employing switching devices having permanentlypaired active and passive contacts for interconnect-- ing subscriberstelephone lines, a holding and release trunk comprising a continuousnon-inductive path extending through each switching stage formaintaining energized the switching devices thereat.

4. In a telephone system employing switching devices having a simplemovement of slight extent for interconnecting subscribers telephonelines, a holding and release trunk comprising a continuous pathextending through each switching stage for maintaining energized theswitching devices thereat.

5. In a telephone system employing switching devices having simplemovements of slight extent for interconnecting telephone lines, arelease trunk comprising a continuous non-inductive path extendingthrough each switching stage.

In witness whereof, I hereunto subscribe my name this 11th day ofFebruary A. D. 1920.

CHARLES L. GO ODRUM.

